A cognitive radio which recognizes an ambient radio environment to optimize a communication parameter according to the ambient radio environment has been known. In an exemplary cognitive radio, there is such a case that a secondary system (interfering system) shares a frequency band allocated to a primary system (interfered system).
While the secondary system shares the frequency band with the primary system, it is required to prevent the secondary system from exerting an adverse effect on an existing service provided by the primary system. Therefore, in a transmitter of the secondary system (hereinafter referred to as “secondary transmitter”), communication is made at a transmission power (hereinafter referred to as “actual transmission power”) adjusted to a value equal to or less than the maximum value of transmission power (hereinafter referred to as “allowed transmission power”), the maximum value of transmission power being capable of keeping a predetermined reception quality in a receiver of the primary system (hereinafter referred to as “primary receiver”). Here, for example, a criterion for keeping the predetermined reception quality is to keep a CIR (Carrier to Interference Ratio) and a CINR (Carrier to Interference plus Noise Ratio) of the primary receiver at a value equal to or more than a predetermined value or to control an interfered amount of the primary receiver to a value equal to or less than a predetermined value.
Non-patent Literature 1 discloses allowed transmission power that controls the CIR at a receiver of a TV (Television) broadcasting system as the primary system to a value equal to or more than the predetermined value. In Non-patent Literature 1, a path loss (propagation loss) at a time when the signal transmitted from the secondary transmitter (secondary signal) arrives at the primary receiver is estimated, and an interfered amount of the primary receiver generated by the secondary signal is further estimated. As a result, allowed transmission power thereof is determined. However, the path loss generated under the actual environment is affected by a planimetric feature around the secondary transmitter, topography between the secondary transmitter and the primary receiver, and shadowing due to the planimetric feature around the primary receiver. This generates an error with respect to a propagation model (path loss estimation of, for example, Okumura-Hata model), resulting in generation of an estimation error of the CIR. Therefore, in Non-patent Literature 1, the allowed transmission power is set by adding a margin corresponding to a degree of the error of the CIR. This ensures constraint of the allowed transmission power corresponding to the degree of the estimation error of the CIR and keeping of the CIR at a prescribed probability. However, if a large margin is required to be set to the allowed transmission power, the allowed transmission power results in decrease.
Further, Patent Literature 1 discloses that a base station device in a radio communication system shares a common and/or neighboring frequency band with another radio communication system, a spaced frequency width between a frequency band to be used of its own and the frequency band to be used by another radio communication system is calculated, an offset distance between an own base station device and a receiving device of another radio communication system is calculated, and, based on the spaced frequency band and the offset distance, the maximum transmission power that would not degrade communication quality of another radio communication system as a priority system is determined Patent Literature 1 further discloses use of the free space propagation loss equation in calculation of the offset distance and the estimated propagation loss amount. Also, in Patent Literature 1, similar to Non-patent Literature 1, presence of the error due to a difference from the actual environment results in generation of an estimation error in the CIR.
Meanwhile, in Non-patent Literature 2, a sensor station located around the primary receiver (disclosed as “receiver of a secondary system (secondary receiver)” in the Literature) measures reception levels of a secondary signal and a signal (primary signal) transmitted from a transmitter of a primary system (primary transmitter) to show an interference monitoring to be used in estimation of the CIR at the primary receiver. In this method, by using the reception levels of the secondary signal and the primary signal that are measured by the sensor station, an estimate value of the path loss used in Non-patent Literature 1 is actually measured and compensated. The actual measurement of the secondary signal ensures estimation of adverse effects of the planimetric feature around the secondary transmitter, topography between the secondary transmitter and the primary receiver, and shadowing due to a planimetric feature around the primary receiver, the adverse effects being causes of the path loss estimation error. Taking the above into consideration, it is possible to compensate the path loss estimate. Similarly, the actual measurement of the primary signal achieves decrease of the estimation error of the path loss between the primary transmitter and the sensor station. This results in decrease of the estimation error of the CIR. Therefore, it is possible to minimize a margin necessary for suppression of the allowed transmission power. This contributes to increase of the allowed transmission power.